The following code, which only works on Windows, crashes with an access
violation somewhere in the garbage collector in a debug build:

>>> import _ctypes
[43470 refs]
>>> _ctypes.COMError(1, 2, 3)

The problem is that the PyComError_Type's tp_flags contains
Py_TPFLAGS_HAVE_GC although the tp_traverse slot is NULL.

The attached patch removes the Py_TPFLAGS_HAVE_GC which is not necessary
at all because a PyComError_Type instance (normally) only contains
simple objects (strings, numbers, None, and a tuple of strings, numbers,
and None), so no reference cycles should be possible.